>Is anyone aware of an effective means to diagnose a tracheal mite infestation?
Yes. You need a microscope and you field strip a bee and open up the trachea and look. That's the only definitive diagnosis.
> I've read that if you have bees walking around outside the hive unable to fly you might have tracheal mites--but how many bees walking around?
They are usually "K" wing, meaning the two wings on each side, which are usually linked, have separated and the back one is in front of the front one. Any significant number would probably do.
>I've always had a few wanderers so I don't know what's normal and what should cause concern.
A few are often worn out workers on their last leg. Usually with frayed wings. Sometimes they are bees with crumpled wings, which are from DWV (Deformed Wing Virus) which is spread by the Varroa mites.
> Do beekeepers treat for tracheal mites as a precautionary measure every fall?
In 34 years I have NEVER treated for tracheal mites. I don't intend to start now. Treating simply perpetuates the problem. Not treating breeds out the problem. The Europeans were almost wiped out by Tracheal mites back in the first half of the 20th century. They did not treat and have bred the problem down to a minor nuisance by simply not treating.
> If so what do you recommend as treatment and how often do you treat?
I don't and I don't.
The "recommended" treatment is menthol. Another that is falling into disfavor becaue of the Small Hive Beetles (SHB) is grease patties.http://www.bushfarms.com/beespests.htm#tracheal